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The European Parliament has called for the immediate release of five Hong Kong booksellers detained in China, in a case that has grabbed global headlines and rattled the diplomatic and business community in the Asia financial centre.

The disappearances have prompted fears that mainland Chinese authorities may be using shadowy tactics that erode the "one country, two systems" formula under which Hong Kong has been governed since its return to China from British rule in 1997.

Chinese police confirmed for the first time on Thursday that three of the five Hong Kong booksellers who went missing were being investigated for "illegal activities" in China.

The booksellers specialize in selling and publishing gossipy political books on China's Communist Party leaders.

The news came two weeks after their colleague, Swedish national Gui Minhai, who disappeared from Thailand in October, appeared on Chinese state television and made a tearful confession to a drink-driving offence more than a decade ago.

A customer holds the Chinese banned book featuring a photo of Chinese President Xi Jinping on the cover at a booth at a market in Hong Kong on Feb. 3. (Kin Cheung/The Associated Press)

Shortly after, Chinese authorities confirmed that Lee Bo, 65, one of the other missing booksellers and a British passport holder, was in China after he vanished from Hong Kong weeks earlier.

Their mysterious disappearances had sparked fears they may have been taken by Chinese agents.

"The resolution calls for their immediate safe release. It also calls for the immediate release of all other persons arbitrarily arrested for exercising their rights to freedom of expression and publication in Hong Kong," the European Parliament said in a statement.

So far, Chinese authorities have not made any substantial statements explaining Beijing's role in the disappearances, nor how the men ended up in China.

Chinese Foreign Ministry spokesman Lu Kang said this week that Chinese law enforcers always abide by the law. Beijing said its law enforcement officials would never do anything illegal, especially not overseas, and that Hong Kong was China's domestic affair and no "foreign country has the right to interfere" in this matter.

The "one country, two systems" formula accords Hong Kong a degree of autonomy and freedoms not enjoyed in mainland China, including freedom of speech.

The British government is still waiting for responses to its diplomatic requests for information and access to Lee.

Authorities including in the European Union and the United States have expressed concerns over the disappearances.